1. Field of the Invention
The related art includes an amplifier arrangement described in European Patent Application EP-A-0,217,431, which includes:
a first and a second output transistor of the same conductivity type, each having a first main terminal, a second main terminal and a control terminal, the first and the second main terminal of the first output transistor being coupled to a first power supply terminal and an output for connecting a load and the first and the second main terminal of the second transistor being coupled to the output and a second power supply terminal,
drive means for driving the first and the second transistor in phase opposition, said drive means having a first input for applying an input signal, a second input, a first output which is coupled to the control terminal of the first transistor and to a power supply terminal by means of a first current source, and a second output which is coupled to the control terminal of the second transistor and to the power supply terminal by means of a second current source,
quiescent current means for causing a quiescent current to flow through the first and the second output transistor, said quiescent current means having first and second current measuring means for generating a first and a second measuring current which are a measure of the current flowing through the first and the second output transistor, respectively, and having feedback means for controlling the quiescent current flowing through the first and the second output transistor by way of the first and the second measuring current.
In the description and the appendent claims the first main terminal, the second main terminal and the control terminal are identical to the drain, the source and the gate, respectively, in the case of unipolar transistors and are identical to the collector, the emitter and the base, respectively, in the case of bipolar transistors.
An amplifier arrangement of this type can be generally used as an output amplifier arrangement and is particularly suitable as an output amplifier in audio circuits.
The first and the second output transistor are push-pull driven by the non-inverting and the inverting output, respectively, of a voltage-to-current converter which has a non-inverting input to which the input voltage is applied and an inverting input which is connected to the output of the amplifier arrangement.
The amplifier arrangement is of the class AB type because a given quiescent current flows through the two output transistors. The means for adjusting the value of this quiescent current comprises a measuring circuit for each of the two output transistors for measuring the current through the relevant output transistor. This measuring current is converted across a first diode into a voltage which is applied to the inverting input of an amplifier. A reference voltage generated across a second diode is applied to the non-inverting input thereof. In the absence of an input signal to drive the output transistors, the output of the amplifier controls the relevant output transistor in such a way that the voltage at the inverting input of this amplifier will become equal to the reference voltage. When driving one of the two output transistors, the quiescent current through the driven transistor decreases and the current through the non-driven transistor increases. The known arrangement is, however, adapted in such a way that this increase cannot prevent the non-driven transistor from being fully turned off. If the relevant transistor is subsequently driven, it should first be turned on, which causes cross-over distortion.